exception handling, thrown errors, within promises

Question

I am running external code as a 3rd party extension to a node.js service. The API methods return promises. A resolved promise means the action was carried out successfully, a f

Answer 1

Promise rejection is simply a from of failure abstraction. So are node-style callbacks (err, res) and exceptions. Since promises are asynchronous you can't use try-catch to actually catch anything, because errors a likely to happen not in the same tick of event loop.

A quick example:

function test(callback){
    throw 'error';
    callback(null);
}

try {
    test(function () {});
} catch (e) {
    console.log('Caught: ' + e);
}

Here we can catch an error, as function is synchronous (though callback-based). Another:

function test(callback){
    process.nextTick(function () {
        throw 'error';
        callback(null); 
    });
}

try {
    test(function () {});
} catch (e) {
    console.log('Caught: ' + e);
}

Now we can't catch the error! The only option is to pass it in the callback:

function test(callback){
    process.nextTick(function () {
        callback('error', null); 
    });
}

test(function (err, res) {
    if (err) return console.log('Caught: ' + err);
});

Now it's working just like in the first example.The same applies to promises: you can't use try-catch, so you use rejections for error-handling.